1. Field of the Invention
The present invention relates to a reproducing amplifier circuit for a magnetic head with a Hall element.
2. Description of the Prior Art
FIG. 1 shows a simple equivalent circuit of a Hall element, which comprises Hall current terminals 1 and 2, Hall voltage terminals 3 and 4, Hall current resistors 5 and 6, Hall voltage resistors 7 and 8 and Hall output voltage sources 9 and 10. When current I.sub.H is fed between the Hall current terminals 1 and 2 and a magnetic field with a magnetic flux density B is applied in the direction perpendicular to the terminals 1 and 2 and across the drawing, the Hall element produces between the Hall voltage terminals 3 and 4 in the direction perpendicular to the terminals 1 and 2 a total Hall output voltage V.sub.H given by the equation (1) EQU V.sub.H =R.sub.H BI.sub.H ( 1)
where R.sub.H is a Hall coefficient. The Hall output voltages are produced by the sources 9 and 10 and are induced with the polarities as shown in FIG. 1.
Therefore, it is necessary to balance-drive the Hall current or to balance-amplify the Hall output voltage. When such a Hall-effect element or device is used in the magnetic head, the current drive element must be a large power supply element in order to increase the Hall current as much as possible and to improve the sensitivity for the purpose of improving the signal-to-noise (S/N) ratio. Therefore, it is unpreferable to balance-drive the Hall current. For this, it has commonly been employed a method that the Hall currents are subjected to unbalanced drive by grounding either one of the terminals 1 and 2, while the Hall output voltages are subjected to balanced amplification by grounding neither of the terminals 3 and 4.
FIG. 2 shows a conventional playback amplifier circuit for amplifying the Hall output voltage of this type Hall-effect element. The Hall voltage amplifier circuit comprises differential input terminals 11 and 12 an amplifier output terminal 13, a power supply terminal 14, transistors 15 and 18, resistors 19 to 27 and capacitors 28 to 32. The input terminals 11 and 12 shown in FIG. 2 are coupled with the Hall voltage terminals 3 and 4 shown in FIG. 1, respectively. The Hall current terminal 2 is connected to ground. The Hall current terminal 1 is connected to a DC current supply circuit (source). In operation, signals inputted into the differential input terminals 11 and 12 are subtracted one from the other since transistors 15 and 16 are coupled to each other in a differential manner. The subtracted signal is amplified by the transistor 18 and then is negatively fed back through the resistor 25 to the transistor 16. Accordingly, Hall output voltages 9 and 10 shown in FIG. 1 and thermal noise due to the Hall voltage resistors 7 and 8 are amplified and are developed at the output terminal 13. Thermal noises produced by the Hall current resistors 5 and 6 do not appear at the output terminal 13 since they are cancelled to each other. The gain of the amplifier is determined by the ratio between the resistor 25 and the Hall voltage resistor 8. As mentioned above, the amplifier circuit is advantageous in that, while the noises caused by the Hall current resistors are cancelled from each other, the balanced output signal is amplified. But it is disadvantageous in that, since two transistors 15 and 16 are used in differential connection, noise generated by the transistors per se is increased by 3 dB, compared to the case of using a single transistor. Particularly, when the resistances of the Hall voltage terminal resistors 7 and 8 are small, the noise generated by the transistor per se is problematic. For example, when the resistances of the Hall voltage terminal resistors 7 and 8 are in the order of 100 to 150 ohms, these resistances are substantially equal to the base diffusion resistance of the transistor, so that it is impossible to design the circuit with a noise figure (NF) of 3 dB or less. In this case, low-noise circuit design therefore needs balanced drive for the Hall currents and unbalanced amplification for the Hall output voltages. Therefore, the Hall current drive circuit has to be of the balanced drive type using large power supply element as mentioned above.